This invention generally relates to the field of communication systems having soft handoff capability and, more particularly, to reducing interference by restricting the application of soft handoff under certain circumstances.
A simplified layout of a cellular communications system is depicted in FIG. 1. Mobile stations M1–M10 communicate with the fixed part of a public switched telephone network (PSTN) by transmitting radio signals to, and receiving radio signals from, cellular base stations B1–B10. The cellular base stations B1–B10 are, in turn, connected to the PSTN via a Mobile Switching Center (MSC). Each base station B1–B10 transmits signals within a corresponding area, or “cell” C1–C10. Within each cell, a base station transmits to the mobile units over downlink RF channels, while the mobile units transmit information to the base station over uplink RF channels.
While cellular systems were originally designed to operate with a one-to-one correspondence between a mobile station and an associated base station covering a geographic cell, it has been determined that the effects of shadowing and fading can be reduced by communicating the same signal to a mobile station over more than one link. For instance, two different base stations can communicate the same information to a mobile station over two different spatially offset links. The mobile station processes the signals from the two links by selecting or combining them in some way, e.g., maximal ratio combining. This technique is known as diversity. Conventional spatial diversity techniques employ two or more separated antennas in a single base station, or two or more base stations, to communicate with a mobile station. However, diversity is not limited to spatially offsetting base stations or antennas (i.e., multiple transmission paths). Diversity transmission can also be generated using one or more of an offset in time, polarization, or frequency.
One area in which macro diversity is commonly practiced is during handoff. In such cases, the candidate base station (i.e., the base station to which a mobile station is to be handed off) starts transmitting substantially the same message information to the mobile station before the current, serving base station terminates its transmission of that message information. This usage of macro diversity is commonly referred to as soft handoff.
FIG. 2 illustrates a soft handoff arrangement wherein a first, original base station 202 and a second, candidate base station 204 each transmit a same message 206 to a mobile station 208. The message 206 is transmitted to the mobile station 208 over different signal paths in the forms of a first downlink 210 and a second downlink 212. The first and second downlink signals 210 and 212 are recombined (or one of the received signals is selected) in the mobile station 208 to extract the message 206. The mobile station 208 transmits to the base stations 202 and 204 over first and second uplink paths 214 and 216, respectively. At some point in time, the transmission of message information to the mobile station from the first, original base station 202 is terminated and the soft handoff process is concluded.
Soft handoffs can also be performed using multiple transmissions from a single base station. FIG. 3 depicts a single base station macro diversity arrangement wherein first and second directional lobes 318 and 320, generated by an antenna array 304, each cover a different area. The mobile station 308 can be handed off from one lobe to another in the manner described above. That is, the first directional lobe 318 maintains a first diversity link including a first downlink 310 which carries a message 306. The second directional lobe 320 establishes a second diversity link including a second downlink 312 which also carries the message 306. First and second uplinks 314 and 316 communicate from the mobile station 308 to the antenna array 304 within each lobe 318 and 320, respectively. Again, at some point in time transmissions from the original lobe will cease to complete the handoff. The same approach can be used with base stations having directional or sector antennas, wherein a soft handoff can be performed between sector antennas, which technique is sometimes referred to as “softer handoff”.
In a soft handoff arrangement, the base stations and/or antennas communicating with a particular mobile station are known as “active set” members. For example, referring back to FIG. 2, base stations 202 and 204 would be considered members of the active set. Those skilled in the art will appreciate that more than two base stations and/or antennas can be part of the active set. Members of an active set can change as the mobile station passes into and out of coverage areas handled by base stations and/or antennas in the system.
Soft handoff has been used in many different types of radiocommunication systems, including those using time division multiple access (TDMA) and code division multiple access (CDMA). Soft handoff increases robustness, achieves improved downlink quality, and combats fading. However, soft handoff may sometimes negatively impact system capacity and network resources due to the additional transmitting source(s) used to transmit substantially the same information to a receiver.
Conventional soft handoff systems ordinarily utilize the same amount of downlink transmit power for each antenna in the active set. For instance, in IS-95 systems, the same transmit power level is used for all downlinks in the active set. Because of the undesirable interference to other users, careful consideration is required in adding and deleting members from the active set so that the interference in unrelated links is minimized. Accordingly, one method for controlling interference is to limit the number of base stations and/or antennas in an active set. This technique is described, for example, in published International Patent Application (PCT) WO/95/12297 to Gilhousen et al., wherein sectors involved in a softer handoff are monitored for reverse link signal strength. If the signal strength of transmissions from one of the sectors drops below a predetermined threshold for a predetermined period of time, then the base station discontinues transmissions from that sector.
Another method used in conventional systems to reduce the effects of unnecessary interference from soft handoff/macro diversity operation is power split control. In power split control, the downlink transmit power may be equally split between each active base station and/or antenna in an active set. That is, in the case where there are three downlinks, and a total transmission power of P is available, each of the downlinks has a transmission power level of P/3. However, even with such an allocation, there may be an unnecessary amount of interference introduced when the “weakest” downlink in the active set is operated at a P/I power level. More specifically, the link may, in effect provide a small improvement in communications robustness, but introduce, on balance, a greater amount of disruption to surrounding communications by unduly introducing interference. Consequently, the C/I ratio for adjacent cells can be negatively impacted with only a minimal gain in communications efficiency.
Recently, Applicants have recognized that some traffic models suggest that a majority of calls in radiocommunication systems, e.g., on the order of 70 percent, are made by mobile units which are relatively stationary. This phenomenon may be attributable to the rapid decrease in size of mobile units over the years, which now permits users to readily carry mobile units with them, as opposed to the earliest mobile units which were large enough to warrant leaving them in a vehicle. If a large number of these stationary calls are made from locations within a cell which trigger the system's soft handoff function, then it may be the case that a large number of stationary mobile units remain in soft handoff mode for the duration of one or more consecutive calls. That is, since the mobile units are stationary, the mobile units will continue to receive transmissions from multiple transmitting sources without being handed off. As described above, this will have a negative effect on the overall system capacity. Moreover, the mobile unit will not always reap significant, additional diversity gains from being in soft handoff mode when stationary.
Accordingly, it would be desirable to provide methods and systems for limiting the application of soft handoff in situations where mobile units are stationary to increase system capacity.